Reduced nitrogen contained in wastewater when discharged into receiving streams exerts a long term oxygen demand which consumes the oxygen resource of the receiving water upon biological oxidation. Both reduced and oxidized nitrogen fertilize receiving waters and are often responsible for algal blooms in lakes. Oxidized nitrogen in the nitrate form has been linked to methemoglobinemia (so called blue babies), a serious disease of infants.
Traditional forms of nitrogen removal from wastewater consist of chemical-physical and biological means. In physical-chemical methods the pH of the wastewater is adjusted to in excess of 9.0 and air is passed through the liquid to remove the ammonia nitrogen. Ammonia stripping is a viable process when volumes are small and ammonia concentrations relatively high. However, for application to wastewater the method has the disadvantage of requiring large quantities of stripping gas with poor performance at low temperature.
Alternatively wastewater is passed through an ion exchange bed. Specific ion exchange media for ammonium ion can be used. Such a method has the disadvantage that organic nitrite and nitrate nitrogen are not removed. Large quantities of regenerant are required and adsorption capacity decreases over numerous regeneration cycles requiring replacement of the ion exchange medium.
Ammonia nitrogen can be removed by break point chlorination. This method has the disadvantage of requiring close pH control and chlorine addition significantly increases the dissolved solids in the wastewater.
Nitrate ion can be removed by ion exchange but the selective resins require scarce petro-chemical feed stock for synthesis and in application require large quantities of corrosive regenerants such as hydrochloric acid.
Nitrogen can be effectively removed biologically by first oxidizing the reduced ammonia and organic nitrogen to nitrate nitrogen followed by biological reduction of the oxidized nitrogen to elemental nitrogen which is given off as a gas.
Domestic sewage contains organic and inorganic nitrogenous material as well as carbonaceous material. For example, a typical raw sewage contains approximately 250 mg/l five day biological oxygen demand (BOD.sub.5), and 40 mg/l total Kjeldahl nitrogen (TKN) of which approximately 30 mg/l is in the ammoniacal form (NH.sub.3 or NH.sub.4 +). Conventional primary sedimentation will reduce the BOD.sub.5 and TKN to about 175 mg/l and 32 mg/l, respectively. Subsequent aerobic biological treatment by, for example, activated sludge under suitable operating conditions oxidizes the ammoniacal nitrogen to nitrite and nitrate nitrogen as well as substantially reducing the BOD.sub.5. Subsequent treatment in a stage containing heterotrophic bacteria where no oxygen is added anaerobic conditions) (anaerobic sufficient organic carbon is present results in reduction of nitrate nitrogen to elemental nitrogen which is given off in gaseous form.
Organisms responsible for oxidation of carbonaceous organic material are ubiquitous and are generally considered to be largely heterotrophic organisms such as zooglea, pseudomonas and chromobacterium which require organic carbon as a food and energy source. Organisms responsible for nitrification are classed as chemotrophic because of their ability to fix inorganic carbon (CO.sub.2) as their carbon source. Nitrosomonas and nitrobacter are representative of the group responsible for nitrification. Denitrification is accomplished by facultative organisms capable of utilizing the oxygen in the nitrate form. Schematically the various transformations are represented as follows:
Heterotrophic Organisms Organic C + O.sub.2 .fwdarw. CO.sub.2 + H.sub.2 O + Cells Nitrosomonas 2NH.sub.4 .sup.- + 3O.sub.2 .fwdarw. 2NO.sub.2 .sup.- + 2H.sub.2 O + 4H.sup.+ Nitrobacterium 2NO.sub.2 .sup.- + O.sub.2 .fwdarw. 2NO.sub.3.sup.- Facultative 2NO.sub.3 .sup.- + Organic C .fwdarw. N.sub.2 + 3CO.sub.2 Heterotrophic Organisms
In conventional biological nitrification and denitrification systems the growth rate of the organisms responsible for nitrification is much slower than the heterotropic organisms. Thus long cell residence times are required to maintain a viable nitrifying mass in order to prevent washing out of nitrifiers either in the effluent or in the wasted sludge. The nitrification rate is strongly dependent upon pH, the optimum value lying between 7.5 and 8.5. Oftentimes it is necessary to add alkalinity to sewages deficient in alkalinity in order to maintain the pH in the optimum region for growth of nitrifiers. The principles governing the above phenomenon are described in a paper by Downing et al. (J. Inst. Sew. Purif., 1961, p. 130).
Denitrification is not only dependent upon the mass of denitrifying organisms present in the system, but also on the availability of organic carbon to provide energy and to act as electron donor or oxygen acceptor in the denitrification step. In practice the denitrification rate is accelerated by providing an organic carbon source, such as methanol, to maintain the denitrification rate at a high level.
Accordingly, it is the object of this invention to provide a method for simultaneously removing organic carbonaceous material and nitrogenous material from sewage under improved conditions so as to accelerate the rate of removal of nitrogen.
A further objective of the invention is to accelerate the rate of denitrification by providing improved conditions for increasing the denitrification rate in the denitrification step.
It is the purpose of this invention to provide a suitable oxygen acceptor which may be substituted for acetate, methanol or other commercial organic biodegradeable material used as an oxygen acceptor. It is a further purpose of this invention to provide for removal and recovery of a portion of the nitrogen for a fertilizer. A still further purpose is to provide suitable alkalinity for maintaining the pH of the nitrifying step. It is still a further purpose of this invention to provide an economical means of sludge disposal while at the same time providing the advantages listed above.